Process for making gypsum board

ABSTRACT

A PROCESS OF PRODUCING GYPSUM OBARD OF ENHANCED STRENGTH AND RESISTANCE TO HUMIDIFIED BOND FAILURE AT ANY PARTICULAR DENSITY THROUGH THE USE OF A DELAYED ACTION ACCELERATOR. THE ACCELERATOR IS PREFERABLY FORMED BY GRINDING LANDPLASTER AND A SUGAR.

y 1974 w. A. KINKADE ET AL 3,813,312

moor-ass FOR MAKING GYPSUM BOARD iginal Filed Oct. 5. 1970 FIG FIG

FIG. 4

FIG. 3

United States Patent Oflice U.S. Cl. 156-39 20 Claims ABSTRACT OF THEDISCLOSURE A process of producing gypsum board of enhanced strength andresistance to humidified bond failure at any particular density throughthe use of a delayed action accelerator. The accelerator is preferablyformed by grinding landplaster and a sugar.

The present invention relates to gypsum board, sometimes called plasterboard, which consists of a gypsum core encased in paper cover sheets.The invention is particular directed to the production of such board ofreduced weight or density without sacrifice in strength and also to theproduction of such board without loss of bond between the paper coversheets and the core on exposure to high humidities, particularly forshort periods of time. In general, the invention is an improvement overthe post acceleration disclosed in Lane et al. US. Pat. 3,359,146, whichpatent, in general, seeks to accomplish at least the former of thesedesirable objectives. Additionally, the invention is intended forapplication, not only to the primary-secondary mixer combinationdiscussed by Lane et al., but also to the more modern multi-pass mixersof the sort described in Camp US. Pat. 2,660,416.

This is a divisional of application Ser. No. 78,067, filed Oct. 5, 1970,now abandoned.

BACKGROUND OF THE INVENTION Gypsum board or plaster board has long beena large volume commercial article of commerce. In addition to the Laneet al. and Camp patents hereinabove referred to, the manufacture ofgypsum board is discussed in Roos Pats. Nos. 2,017,022 and 2,080,009. Ingeneral terms gypsum board is manufactured by dispersing calcined gypsumin water and adding thereto a light weight pregenerated foam to controlthe finished density of the slurry. Additives conventionally used inminor amounts include accelerators, bond protecting agents, fibrousreinforcements, and consistency reducers. Typical of accelerators arecalcium sulfate dihydrate, potassium sulfate, ammonium sulfate, andaluminum sulfate. Bond protecting agents are unusually cereal flours orstarches. The fibrous reinforcements may be either cellulosic or glass.Consistenc'y reducing agents are typified by the lignosulfonates, ofwhich ammonium lignosulfonate is particularly advantageous. Theseadditives are used in minor quantities in relation to the total weightof the board core, and represent, in total, less than usually less than2%, of the weight of the finished core.

The slurry containing the desired ingredients is prepared in continuousmixers, such as the primary-secondary mixer combination described byLane et al. or the multi-pass mixer described in the above mentionedCamp patent. The mixed slurry is continuously deposited on a paper coversheet moving beneath the mixer. A second paper cover sheet is appliedthereover and the board is passed under a roll or rolls to adjust thethickness. The continuous strip thus formed is conveyed on a belt untilthe calcined gypsum has set, whereafter the strip is cut 3,813,312Patented May 28, 1974 to form boards of desired length and the boardsare conveyed through a drying kiln to remove excess moisture.

The Lane et al. solution to some of the problems to which the presentinvention is addressed involves, in general, a post-accelerationprocess, in which the calcined gypsum feed is dispersed in water to ahigh degree of fineness, whereafter the accelerator is added to theslurry and the product is then cast. It is clear from the Lane et al.disclosure that a high degree of dispersion (i.e. fineness) of thecalcined gypsum is highly advantageous in producing a board of maximumstrength. The Lane et al. process, insofar as commercial gypsum boardoperations are concerned, is directed to a primary-secondary mixer andfoam and accelerator are added to in the secondary mixer. Although thisoperation works quite well in that mixer combination, it is notconveniently adapted to the more modern multi-pass mixer described byCamp. In the latter, the peripheral velocity increases as the materialmoves toward the outer rim of the mixer, thus making the final mixingmore intense, rather than less intense, as is the case with theprimary-secondary combination.

In considering the strength, as measured by compres sive breaking loadof gypsum board or gypsum board core in relation to the density thereof,the standard or normal strength concept described by Lane et al. hasbeen found highly useful. Lane et al. point out that thestrength-density relationship is not a linear one, but insteadcorresponds to the equation S=A+10 wherein S is the compressive strengthin pounds per square inch, A is a constant amounting to 29.02 and D isthe density in pounds per cubic foot.

Another characteristic of gypsum board important in the presentinvention is what is known as humidified bond. Brief exposure to highhumidities, such as a test condition involving one hours exposure at F.and 90% relative humidity, causes a weakening at or near the paper-coreinterface, as a result of which the paper may be partially or completelystripped from the core with a relatively gentle pull.

On failure, a little of the core, perhaps up to a few thousansd of aninch in thickness, is often removed with the paper. This is perhaps dueto strains set up by partial penetration of high moisture content airinto the board, well short of equilibrium. This condition also occurs instorage and use of the board when it is exposed to varying ambienttemperature and humidity conditions. In general, the lower densityboards exhibit greater humidified bond failture than do the higheddensity boards.

OBJECTS OF THE INVENTION It is a primary object of the present inventionto provide a process for making gypsum board of lower density andlighter weight than heretofore commercially achieved without sacrificein strength of the board or its resistance to bond failure onhumidification.

It is a further object of the invention to accomplish these desirableresults in board manufactured on conventional gypsum board producingmachinery.

It is a still further object of this invention to accomplish theaforesaid desirable results in board made on commercial machinerywithout sacrificing production speed.

It is a still further object of this invention to accomplish theaforesaid advantageous results by the use of a delayed actionaccelerator.

It is a still further object of this invention to control the hydrationof the calcined gypsum in a fashion which provides relatively slowtemperature rise during the early portions of the hydration period.followed by a rapid temperature rise toward the end of the temperaturerisemeasured hydration period.

GENERAL DESCRIPTION OF THE INVENTION We have found that the foregoingand other desirable objects are achieved by incorporating into thecalcined gypsum slurry as the sole accelerator of the set thereof anaccelerator whose accelerative elfect is a delayed one. Suchaccelerators are preferably sugar coated landplaster (calcium sulfatedihydrate) of the sort disclosed, for other purposes, in King US. Pat.2,078,199.

GENERAL DESCRIPTION OF THE DRAWINGS FIG. I is a scanning electronmicroscope photograph of a laboratory-made set gypsum cast preparedusing the set stabilization composition taught in King Pat. 2,078,199.

FIG. 2 is a scanning electron microscope photograph of a laboratory-madeset gypsum cast using the delayed action accelerator discolsed in thisinvention, without the retarder called for by King.

FIG. 3 is a scanning electron microscope photograph of the core of acommercial gypsum board manufactured using Microfloc as an accelerator(see McCleary and Kinkade Pats. Nos. 3,262,799 and 3,307,919).

FIG. 4 is a scanning electron microscope photograph of the core of acommercial gypsum board made in accordance with this invention.

DETAILED DESCRIPTION OF THE DRAWINGS The insight into themicro-structure of the crystals in a gypsum cast provided by FIGS. 1-4has only recently been possible. The instrumental development which hasmade this insight possible is referred to as scanning electronmicroscopy. The technique is described generally in the followingquotation from an article titled. Morphology of Dental Surfaces andAdhesion of Polymeric Filling Materials: Primer Studies With ScanningElectron Microscopy, by Henry Lee, Michael L. Swartz, and D. G. Stotfeywhich appeared at page 243 of the preprint booklet of the Division ofOrganic Coatings and Plastics Chemistry of the American ChemicalSociety, volume 30, No. 1 (references to citations and figures omittedfrom the quotation):

Inasmuch as there are only a limited number of these new instrumentsavailable, it would seem desirable to digress for a few minutes toreview the scanning electron microscope for those readers who have nothad the occasion to work with one personally.

The scanning electron microscope differs from the transmissioninstrument in that it uses reflected (backscatter) electrons or morepreferably, secondary emission electrons, emitted from the incidentsurface.

The secondary emission electrons are preferred to the backscatterelectron as they provide higher contrast due to their enhanced emissionin the case of rough surface.

In addition, the incident beam of electrons in a scanning microscope isnot stationary but is scanning the specimen surface in a TV rasterpattern. A directly synchronized raster pattern is displayed on acathode ray tube, and is modulated by the signal from the secondaryelectron detector.

The result is a picture displayed on the cathode ray tube which providesmagnifications of 50X to 140,000X, on a resolutionof about 300-400angstroms, and a depth of focus 300 500 times greater than a lightmicroscope or a transmission electron microscope.

The sequence of operations for use of the microscope arestraightforward. The specimens are examined in '4 1; most cases by useof a transmission or reflected-light light microscope to determine theareaof probableinteresta- Then the specimen is mounted on a specimenholder, usually a small brass cylinder, using a conducting(silvercontaining) paint or adhesive. The specimen cylinders are thenmounted on a plastic tray, using double backed adhesive tape, and placedin a vacuum evaporator. A coating of gold, copper, or aluminum about 200angstroms thick is deposited on the specimen. The specimen is rotatingduring coating so that a uniform conducting coating is obtained.Generally, no attempt is made to shadow the specimen as in transmissionmicroscopy, as the purpose is not to create a shadow, but to provide aconductive surface so that electrons will not build up a charge in agiven area and distort the picture. The thickness of coating is lessthan that of the resolution and does not alter the picture but actuallyincreases the sharpness of the image.

A specimen cylinder is then mounted in a specimen boat and inserted inthe forechamber of the instrument and then into the operating chamber.The position ofthe' specimen is adjusted by means of dials which permitX or Y horizontal motion, as well as rotation or tilting. The image isviewed on two cathode ray tubes, located on the control console. A 35mm. camera or Polaroid camera is swung into place to record desirablepictures.

In all of the figures the gypsum cast surface 'is depicted inmagnification of 6420 and the scanning angle in each case was 45. Theelipse shown in the lower righthand corner of each figure indicates a1.95 microns dimension, the major and minor axes of the elipsecorresponding 1to directions parallel and transverse to the mounting ange Examination of FIG. 1 reveals that the King composition is made up ofa multitude of squat, stubby crystals. In contrast, the composition ofthis invention, shown in FIG. 2, consists almost exclusively of veryslender, long, rod-like crystals of diameters less than about 4 micron,with almost complete absence of the squat, stubby crystals found in theKing composition.

That this crystal formation is responsible for the improvedcharacteristics of the board of the present invention is illustrated bya comparison of FIGS. 3 and 4. In FIG. 3, which shows a prior art boardcomposition, it will be seen that at least half the crystals visible aresquat, stubby crystals of the sort shown in FIG. 1. There are only a fewlong rod-shaped crystals visible, and these are, of quite largediameter, on the order of one micron. or more. In contrast, the boardcore composition of this invention, shown in FIG. 4, exhibits theslender, rod-like crystals also shown in FIG. 2. Studdy, squat crystalsare almost completely absent. It will be observed that a sub- I stantialmajority of the crystals of FIG. 4 have a diameter of less than aboutmicron. This dramatic difference in crystal formation and ultimatecrystal size and shape is believed to be responsible for the improvedcharacteristics of the board produced according to the presentinvention. The accelerator of this invention should be present in anamount equal to between about five and about twenty pounds per ton ofcalcined gypsum.

SPECIFIC EMBODIMENTS OF THE INVENTION Example I Parallel runs were madeon a commercial gypsum board machine in which the control board was madewith Micentimeters per gram (see ASTM method C-204). The difference inthe setting behavior is shown in the data in the subjoined table.

It will be observed that the temperature rise during the early stages ofset with the accelerator of the present invention is much less than wasobserved in the control run and that the rate of temperature rise towardthe end of the setting period was substantially greater in the case ofthe delayed action accelerator used in the present invention. The testsrecounted in this example further show that the normal weight of thecontrol board was about 1,900 lbs./m. sq. ft. (45.7 1b./cu. ft. coredensity) and that equivalent strengths and humidified bond performancewere achieved with the delayed action accelerator of the presentinvention at weights as low as 1,675 lbs./m. sq. ft. (39.9 lbs./cu. ft.core density).

A further advantage disclosed in the trials described in this examplelay in the edge hardness of the board. It is well known that the edgesof gypsum board are especially subject to calcination and thereforeexcessive softness in passing through the drying kiln. In the foregoingtests it was found that the average edge hardness went up about 5 points(out of approximately 15-20) at the same board weight when using thedelayed action accelerator of the present invention. Further datacollected in the test recounted in this example are shown in the twosubjoined tables, the first of which recounts the core quality of theboards and the second of which sets forth the results on the humidifiedbond test after one hours humidification at 90 F. 90% RH.

TABLE I-B Dry density Percent lbs] P.s.i. normal cubic it. strengthstrength Control, microfloc accelerator 47. 5 658 83. 0 Test, delayedaction accelerator normal" weight board 46. 8 641 105. 0 Test, delayedaction accelerator reduced weight board 45. 4 674 105. 0 Do 42. 6 52098. 0 Test, delayed action accelerator normal weight board 46. 7 769110. 0

TABLE I-C Percent bond Number V board Core failure of boards weight,density, in average lbs./m. 1bs./cu. it. Face Back Control, microfiocacceleration 6 1, 868 45. 0 28 69 Delayed action accelerator 8 1, 89245. 6 2 3 D0 6 1, 797 43. 0 7 12 Do 4 1,815 43.6 6 30 Do 6 1, 720 41. 035 81 Example [II The tests reported in this example were conducted at acommercial gypsum board producing facility different from that ofExample I and indicate the same sort of range of improvement. The datais given in the subjoined table.

TABLE II Block and Delayed potassium action sulfate acceler- Test unitsaccelerator ator Us e LbJm.

ag sq. ft 10+1. 5 7 Final set temperature rise Minutes" 6. 5 6. 8Maximum rate of hydration. F./ minute-.- 7. 6 8. 45 Temperature riseduring 3d F 5. 2 4. 8

minute. Ratio of maximum rate to rise during 3d minute. 1. 46 1. 76

The above Table shows that by the use of the delayed action acceleratorof this invention in the process for forming gypsum wallboard of thisinvention, the maximum rate of temperature rise during set of the corein the manufacture of gypsum wallboard is at least about 1.5 times thetemperature rise occurring during the third minute, and the maximum rateof temperature rise occurs subsequent to the third minute.

Example III A series of gypsum boards was prepared in the laboratory tocompare the performance of conventional ground block accelerator withthe delayed action accelerator of this invention. The latter was thepreviously described landplaster which had been ground with 5% ofsucrose. The boards were prepared at a series of core densities from anupper value of 50 lbs. per cubic foot to a lower value of 38 lbs. percubic foot. Quite in contrast to commercial plant experience, groundblock, carefully prepared under laboratory conditions, performs quiteeffectively as an accelerator. The boards at corresponding densitieswere found to be substantially equal in core compressive strength and inresistance to passage of air, the latter as measured on a Gurleydensometer. However, the test for humified bond failure clearly shows,even under these nearly ideal conditions, the significant advantages ofthe present invention, particularly when it is desired to'rnake lowerweight boards. The boards were conditioned for 24 hours at F. and 90% RHand then tested in the usual bond failure test. The density and thepercent bond failures are shown in the subjoined table.

It will be observed that at high core densities the humidified bond issubstantially equal as between ground block accelerated board and boardaccelerated with the delayed action accelerator of this invention.However, very marked and important differences begin to occur as thedensity of the core is reduced. The results at the lower density rangesare especially significant. Note that at 39.5-40 lbs. density theaccelerator of this invention produced a board showing only 20% bondfailure and that the percent failure increased only to 53% when thedensity was further reduced to 38-39 lbs. In contrast, the boardprepared with ground block accelerator showed 57% bond failure at39.5-40 lbs. density and 97% bond failure at 38-39 lbs. density. Thisillustrates the extreme criticality of this phenomenon as efforts aremade to reduce the board weight.

The data in this example, from boards made under controlled laboratoryconditions, also illustrates, when compared to the bond failure datarecorded in Example I, the differences expectable on translation oflaboratory experionce to full scale production machinery. The dataclearly shows the highly advantageous results of the delayed actionaccelerator of the present invention in the manufacture of lightweightboard.

We claim:-

1. In a process of producing paper covered plasterboard characterized byenhanced strength at any particular density and by improved bond of coreto paper the improvement which comprises admixing calcined gypsum,water, and pregenerated foam together in the presence of a delayedaction accelerator, said accelerator being the uncalcined product ofgrinding landplaster with up to about of its weight of sugar and beingpresent in an amount sufficient to produce a temperature rise set ofsaid calcined gypsum in not more than about minutes.

2. The process of claim 1 wherein said accelerator has a Blaine finenessof at least about 8,000 sq. cm./ g.

3. The process of claim 1 wherein the maximum rate of temperature riseduring set is at least about 1.5 times the temperature rise occurringduring the 3rd minute, and said maximum rate af rise occurs subsequentto said 3rd minute.

4. The process of claim 1 wherein said delayed action acceleratorcomprises an intimate dispersion of finely ground landplaster and asugar.

5. The process of claim 1 wherein said accelerator is present in anamount equal to between about 5 and about pounds thereof per ton ofcalcined gypsum.

6. The process claimed in claim 5 wherein said accelerator is present inan amount equal to at least about ten pounds thereof per ton of calcinedgypsum.

7. The process claimed in claim 1 wherein the said accelerator ispresent in an amount equal to between about 5 and about 20 poundsthereof per ton of calcined p 8. A method for increasing the strength atany particular density of the gypsum core of a wallboard formed fromstucco, the method comprising the steps of incorporating a delayedaction accelerator comprising a finely ground uncalcined mixture oflandpl'aster and a sugar in the stucco, said sugar being present in anamount up to about 5% of the weight of said landplaster, and thereaftercasting the stucco between paper cover sheets and permitting it to setto form gypsum wallboard.

9. In a process of producing paper covered plasterboard, whereincalcined gypsum, water, and a pregenerated foam are admixed together toform a slurry, and said slurry is disposed between paper sheets andallowed to set, the improvement comprising admixing a delayed actionaccelerator with said calcined gypsum, water and pregenerated foam toform said slurry, said delayed action accelerator comprising a finelyground uncalcined mixture of landplaster and sugar, said sugar beingpresent in an amount up to about 5% by weight of said landplaster, theamount of said delayed action accelerator being sufficient to produce atemperature rise set of said calcined hypsum in not more than about 15minutes, whereby said plasterboard has enhanced strength at anyparticular density.

10. The improvement in'a process according to claim 9, wherein saiddelayed action accelerator has a Blaine fineness of at least about 8000sq. cm./g.

11. The improvement in a process according to'c-laim 9, wherein themaximum rate of temperature rise during setis at least about 1.5 timesthe temperature-rise occurring during the third minute, and said maximumrate of rise occurs subsequent to said third minute.

12. The improvement in a process according to claim 9, wherein theamount of said delayed action accelerator admixed with said calcinedgypsum, water and pregenerated foam is equal to between about 5 andabout 20 pounds per ton of calcined gypsum.

13. The improvement in a process according to claim 12, wherein theamount of said delayed action accelerator which is admixed with saidcalcined gypsum, water and pregenerated foam is equal to at least about10 pounds per ton of calcined gypsum.

14. The improvement in a process according to claim 9, wherein saidsugar is sucrose.

15. In a process of producing'plasterboard, wherein calcined gypsum,water, and a pregenerated foam are admixed together to form a slurry,and said slurry is formed into a sheet and allowed to set, theimprovement comprising admixing a delayed action accelerator with saidcalcined gypsum, water and pregenerated foam to form said slurry, saiddelayed action accelerator comprising a finely ground uncalcined mixtureof landplaster and sugar, said sugar being present in an amount up toabout 5% by weight of said landplaster, the amount of said delayedaction accelerator being sufrficient to produce a temperature rise setof said calcined gypsum is not more than about 15 minutes, whereby saidplasterboard had enhanced strength at any particular density.

16. The improvement in a process according to claim 15, wherein saiddelayed action accelerator has a Blaine fineness of at least about 8000sq. cm./g.

17. The improvement in a process according to claim 15, wherein themaximum rate of temperature rise during set is at least about 1.5 timesthe temperature rise occurring during the third minute, and said maximumrate of rise occurs subsequent to said third minute.

18. The improvement in a process according to claim 15, wherein theamount of said delayed action accelerator admixed with said calcinedgypsum, water and pregenerated foam is equal to between about 5 andabout 20 pounds per ton of calcined gypsum.

19. The improvement in a process according to claim 18, wherein theamount of said delayed action accelerator which is admixed with saidcalcined gypsum, water and pregenerated foam is equal to at least about10 pounds per ton of calcined gypsum.

20. The improvement in a process according to claim 15, wherein saidsugar is sucrose.

References Cited UNITED STATES PATENTS 3,359,146 12/1967 Lane et al.156-43 2,078,199 4/1937 King 106-111 X 2,007,315 7/1935 Turner l06114DOUGLASS J. DRUMMOND, Primary Examiner D. H. SIMMONS, Assistant ExaminerUS. Cl. X.R. l061 14

